This invention relates to a method and apparatus for magnetic resonance signal acquisition and a magnetic resonance imaging apparatus, and more particularly relates to a method and apparatus for magnetic resonance signal acquisition for magnetic resonance imaging based on a phase contrast method and a magnetic resonance imaging apparatus for magnetic resonance imaging based on the phase contrast method.
A magnetic resonance imaging (MRI) apparatus generates a magnetic resonance signal by applying a gradient magnetic field and a high frequency magnetic field wherein in the magnetic resonance imaging apparatus an imaging target has been placed in the internal space of a magnet system, namely the space where a static magnetic field is formed, and forms (restructures) a tomographic image based on the received signal.
The phase contract method has been known as one of the method for imaging the flow of blood or the like in an organism. In this method, the gradient magnetic field that does not change the phase of the spin the position of which is not changed during imaging, and the gradient magnetic field that changes the phase of only the spin the position of which is changed during imaging is used. Such a gradient magnetic field is called flow encode gradient magnetic field.
The flow encode gradient magnetic field is the method in which two magnetic fields having inverse gradients are applied for an equal time successively. By applying such a gradient magnetic field, the phase of the spin of no positional change changes in one direction in the first half time of the gradient magnetic field application period and changes in the inverse direction in the second half time by the same magnitude. The phase of the spin of no positional change does not change after all. On the other hand, the phase of the spin of positional change along the gradient of the magnetic field changes in one direction in the first half time of the gradient magnetic field application period but changes in the second half time by the different magnitude because the intensity of the magnetic field is different between first half and second half due to the positional change. The phase of the spin of positional change does not return to the original phase and causes the change from the original phase.
In the case that the flow contrast method is applied, two gradient magnetic fields that allow the spin of positional change to cause inverse phase changes at different timings are used as a pair of flow encode gradient magnetic field. The difference between the magnetic resonance signal obtained under the one flow encode gradient magnetic field and the magnetic resonance signal obtained under the other flow encode gradient magnetic field is calculated in order to offset the magnetic resonance signal is the spin of no positional change, and only the magnetic resonance signal of the moving spin remains there. By restructuring of an image based on the abovementioned magnetic resonance signal, an image of the moving spin, namely for example an image of a blood flow image, can be obtained.
However to say strictly, the phase of the no positional spin causes the change due to Maxwell term arising from the flow encode gradient magnetic filed. Because the phase change due to the one flow encode gradient magnetic field of the pair is generally different from the phase change due to the other flow encode gradient magnetic field of the pair, and the phase change signal of the spin of no positional change remains residual in the difference of the magnetic resonance signal. Therefore, the flow image includes an image of a static portion as a residual image. The lower static magnetic field gives the larger Maxwell term, and the residual image is more remarkable on the flow imaging in the lower magnetic field application.
Therefore, it is the object of the present invention to realize a method and an apparatus for magnetic resonance signal acquisition for acquiring a magnetic resonance signal having a small Maxwell term, and to realize a magnetic resonance imaging apparatus having the abovementioned magnetic resonance signal acquisition apparatus.
(1) An aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition method for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized in that a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(2) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition method for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized in that a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(3) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition method for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized in that a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field, and a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that nave the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(4) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition method described in any one of (1) to (3) characterized in that the first half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field, and the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, in addition to the abovementioned (1) to (3), the first half of the flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin is rephased by means of the slice gradient magnetic field, and the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field, as the result the Maxwell term is reduced to a small value.
(5) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal of a target by means of phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(6) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(7) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field and, a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0.
(8) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus described in any one of (5) to (7) characterized by comprising a gradient magnetic field application means in which the first half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field and, the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, in addition to the abovementioned (5) to (7), the first half of a flow encode gradient magnetic field having the first half and second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin is rephased by means of the slice gradient magnetic field, and the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field, as the result the Maxwell term is reduced to a small value.
(9) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field and for structuring an image based on the acquired magnetic resonance signal, characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and as the result the Maxwell term can be reduced to 0. By doing the above, a flow image without a residual image can be obtained.
(10) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field and for structuring an image based on the acquired magnetic resonance signal, characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0. By doing the above, a flow image without a residual image can be obtained.
(11) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus for acquiring a magnetic resonance signal from a target by means of a phase contrast method by use of a static magnetic field, a high frequency magnetic field, a slice gradient magnetic field, a phase encode gradient magnetic field, a readout gradient magnetic field, and a flow encode gradient magnetic field and for structuring an image base on the acquired magnetic resonance signal, characterized by comprising a gradient magnetic field application means in which a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the readout gradient magnetic field in a time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field, and a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the slice gradient magnetic field in a time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the readout gradient magnetic field direction in a time period coincident with the rephase time period by means of the slice gradient magnetic field, and the flow encode gradient magnetic field is converted to a gradient magnetic field having the first half and the second half that have the gradient direction opposite to each other and that have the same absolute gradient value, and the spin is applied to the slice gradient magnetic field direction in a time period coincident with the dephase time period by means of the readout gradient magnetic field, and as the result the Maxwell term can be reduced to 0. By doing the above, a flow image without a residual image can be obtained.
(12) An another aspect of the invention to be applied to solve the abovementioned problem involves a magnetic resonance signal acquisition apparatus as described in any one of the abovementioned (9) to (11) characterized comprising a gradient magnetic field application means in which the first half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is rephased by means of the slice gradient magnetic field, and the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field.
In the invention of this aspect, in addition to any one of the abovementioned (1) to (3), the first half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin is rephased by means of the slice gradient magnetic field, and the second half of a flow encode gradient magnetic field having the first half and the second half in which the gradient of the respective halves is reversed and having an equal absolute value of the gradient is applied in the direction of the phase encode gradient magnetic field in the time period coincident with the time period when the spin of the target is dephased by means of the readout gradient magnetic field, as the result the Maxwell term is reduced to a small value. By doing the above, a flow image with a very slight residual image can be obtained.
According to the present invention, a method and an apparatus that acquire a magnetic resonance signal with a small Maxwell term is realized, and a magnetic resonance imaging apparatus provided with such a magnetic resonance signal acquisition apparatus is realized.